Generally, devices for producing nitrogen gas by the pressure swing adsorption (PSA) method (namely, nitrogen PSA devices) are used in many applications as simple nitrogen gas supply devices. In recent years, in response to user needs, further reductions in the electric power consumption and space requirements are being demanded of these types of devices for producing nitrogen gas.
Conventionally, known examples of these types of devices for producing nitrogen gas include the device described below. FIG. 9 is a diagram illustrating one example of a conventional device for producing nitrogen gas.
As illustrated in FIG. 9, a device 101 for producing nitrogen gas includes a compressor 102, two adsorption tanks (a first adsorption tank 103 and a second adsorption tank 104), a product tank 105, automatic switching on-off valves 111a, 111b, 112a, 112b, 113a, 113b, 114 and 115 provided at the inlet and outlet of each of the adsorption tanks, a flow regulating valve 116 and a product gas outlet valve 117.
The compressor 102 is a device for compressing the raw material gas (air). Further, the two adsorption tanks 103 and 104 are packed with an adsorbent 109 that preferentially adsorbs the oxygen within the raw material gas. In each of the adsorption tanks 103 and 104, the oxygen within the raw material gas is adsorbed and removed, yielding a product gas rich in nitrogen.
Further, the first adsorption tank 103 and the second adsorption tank 104 are connected via lines provided at the downstream side and upstream side of the respective tanks.
Here, the expression “upstream side of the tank” refers to the lower side (the raw material gas inlet side) of the tank, whereas the expression “downstream side of the tank” refers to the upper side (the product gas outlet side) of the tank, which represents a downstream position in terms of the flow of the raw material gas.
The product tank 105 is provided on the downstream side of the two adsorption tanks 103 and 104, and is connected to the first adsorption tank 103 via the on-off valve 113a, and to the second adsorption tank 104 via the on-off valve 113b. 
One known method for separating nitrogen gas from a raw material gas using this type of PSA device 101 for producing nitrogen gas is a method that involves repeating a pressurization and adsorption step, a depressurization and equalization step, a depressurization and regeneration step, and a pressurization and equalization step.
In the PSA method, the first adsorption tank 103 and the second adsorption tank 104 adopt a relationship such that when one is performing the pressurization and adsorption step, the other is performing the depressurization and regeneration step, and when one is performing the depressurization and equalization step, the other is performing the pressurization and equalization step. Accordingly, in the first adsorption tank 103, when the pressurization and adsorption step, the depressurization and equalization step, the depressurization and regeneration step, and the pressurization and equalization step are performed in that order, in the second adsorption tank 104, the steps are performed in the order of the depressurization and regeneration step, the pressurization and equalization step, the pressurization and adsorption step, and the depressurization and equalization step. The following description describes the steps for the first adsorption tank 103.
First, in the pressurization and adsorption step, the raw material gas that has been pressurized by the compressor 102 is fed into the first adsorption tank 103, the inside of the first adsorption tank 103 is pressurized, and the oxygen within the raw material gas is adsorbed preferentially by the adsorbent 109, yielding a gas rich in nitrogen.
Next, in the depressurization and equalization step, residual gas inside the first adsorption tank 103 is introduced into the second adsorption tank 104.
Subsequently, in the depressurization and regeneration step, the first adsorption tank 103 is opened to the atmosphere, thereby reducing the pressure, desorbing the oxygen from the adsorbent 109 and regenerating the adsorbent 109.
In the pressurization and equalization step, gas is introduced into the first adsorption tank 103 from the second adsorption tank 104.
In a more detailed description based on FIG. 9, when the first adsorption tank 103 is performing the pressurization and adsorption step, the on-off valves 111a and 113a are open, and the other on-off valves are closed.
Accordingly, the raw material gas that has been pressurized by the compressor 102 passes through the on-off valve 111a and is fed into the first adsorption tank 103.
In the first adsorption tank 103, the oxygen within the raw material gas is adsorbed by the adsorbent 109, and the resulting nitrogen-rich product gas passes through the on-off valve 113a and is introduced into the product tank 105.
A portion of the product gas discharged from the first adsorption tank 103 passes through the flow regulating valve 116 and into the second adsorption tank 104, where it is used for regenerating the adsorbent 109. As time passes, the region of the adsorbent in which the oxygen adsorption occurs shifts closer to the product outlet end, and therefore after a prescribed time, the pressurization and adsorption step is halted.
Subsequently, when the first adsorption tank 103 enters the depressurization and equalization step and the second adsorption tank 104 enters the pressurization and equalization step, the on-off valves 114 and 115 are opened, and the other on-off valves are closed.
In this step, the gas (pressure equalization gas) inside the first adsorption tank 103, which is under relatively high pressure and, although not satisfying final product quality, is a comparatively nitrogen-rich gas, is supplied from the first adsorption tank 103 to the second adsorption tank 104 through the on-off valves 114 and 115.
This method in which, in the manner described above, gas is introduced from the product gas outlet side (upper side) of the first adsorption tank 103 into the product gas outlet side (upper side) of the second adsorption tank 104, and from the raw material gas inlet side (lower side) of the first adsorption tank 103 into the raw material gas inlet side (lower side) of the second adsorption tank 104 is termed an upper-lower simultaneous pressure equalization method. This type of upper-lower simultaneous pressure equalization method is disclosed, for example, in Non-Patent Document 1 and Patent Document 1.
Next, by opening the on-off valve 112a of the first adsorption tank 103, the first adsorption tank 103 enters the depressurization and regeneration step. In the depressurization and regeneration step, residual gas inside the tank is released into the atmosphere from the on-off valve 112a, and as the pressure inside the tank decreases, the oxygen adsorbed to the adsorbent 109 desorbs. At this time, a portion of the product gas discharged from the second adsorption tank 104 passes through the flow regulating valve 116 and into the interior of the first adsorption tank 103, and is used for purging the inside of the tank and regenerating the adsorbent 109.
Subsequently, by opening the on-off valves 114 and 115, and closing the other on-off valves, the first adsorption tank 103 enters the pressurization and equalization step, and the second adsorption tank 104 enters the depressurization and equalization step. In this step, the gas (pressure equalization gas) inside the second adsorption tank 104, which is under relatively high pressure and, although not satisfying final product quality, is a comparatively nitrogen-rich gas, is supplied from the second adsorption tank 104 to the first adsorption tank 103.
By repeating the above steps, nitrogen gas is separated from the raw material gas.
Patent Document 2 discloses a device for producing nitrogen gas which improves the basic device for producing nitrogen gas described above and increases the amount of product nitrogen generated. This device for producing nitrogen gas is provided with the two adsorption tanks, and another adsorption tank (hereafter called the “pressure equalization adsorption tank”), which is provided partway along the pressure equalization line provided on the product outlet side of the two adsorption tanks, and is used for adsorbing oxygen molecules within the pressure equalization gas.